Russian President Vladimir Putin has said that the Islamic State (IS) group has seized nearly 700 hostages in part of Syria controlled by US-backed forces, executing some of them and promising to kill more.

Speaking in the Black Sea resort city of Sochi on Thursday, Putin said the hostages included several US and European nationals, adding that IS was expanding its control in territory on the left bank of the River Euphrates controlled by US and US-backed forces.

"They have issued ultimatums, specific demands and warned that if these ultimatums are not met they will execute ten people every day. The day before yesterday they executed ten people," Putin told the Valdai discussion forum.

The president did not specify what the group's demands were.

TASS news agency reported on Wednesday that IS fighters had taken around 700 hostages in Syria's Deir Ezzor province after attacking a refugee camp in an area controlled by US-backed forces on 13 October.

TASS said the group had kidnapped around 130 families and taken them to the city of Hajin.

Rami Abdulrahman, who runs the UK-based Syrian Observatory of Human Rights (SOHR), told Euronews that the number of hostages was actually around 300.

He confirmed they were taken last week from a camp near Hajin, east of Deir Ezzor, and were brought to territory controlled by the militant group.

“The Islamic State was controlling the camp for three hours and took the people hostage from the camp to inside the ISIS area,” he said.

“Inside the camp some people who were refugees and pro-ISIS were helping ISIS to control the camp for three hours and to take the people hostage.”

Abdulrahman said that Belgian and French nationals were among the hostages.

“Their lives are in a very dangerous situation”, he said, although he said there had been no confirmed reports of any fatalities as of Thursday afternoon.

In Washington, the US military cast doubt on Putin's claims, the Reuters news agency reported.

"While we have confirmed that there was an attack on an IDP (internally displaced persons) camp near [Deir Ezzor] last week, we have no information supporting the large number of hostages alleged by President Putin and we are skeptical of its accuracy," Commander Sean Robertson, a Pentagon spokesman, said in a statement.

"We are also unaware of any US nationals located in that camp," Robertson added.